Control or Cancellation (ANC)
We have received many
inquiries about the feasibility of ANC in reducing
domestic and commercial noise problems. The conventional wisdom
seems to be that most noise situations are amenable to an ANC solution.
false, just as is the notion that the computer is the solution to all
calculation and bookkeeping problems, or that computers reduce the
consumption of paper.
Consider that most general noise problems are
3-dimensional in nature, typically a noisy area where one or more
work or relax.
Explanations are needed:
ANC uses the principle of destructive interference, where the noise
wave is sensed with a microphone. This signal is amplified and fed to a
loudspeaker which emits sound of the same type but of opposite phase
with respect to the
original sound wave. Such cancellation can occur at only one point in
space. In the case of one-dimensional
propagation down a duct, the sound wave for all intents is "stopped" at
that point since the sound from the loudspeaker proceeds
one - dimensionally down the duct, matching the original noise waves
along the way. (upstream toward the sound source, the sound wave is
doubled, but that is of no concern.
In the case of two or three dimensional propagation into
space as into a room, The
spreading of the loudspeaker sound does not match the pattern of the
divergence of the noise field, having been produced a longer distance away. for
widespread cancellation to occur,
that the loudspeaker be precisely placed at the center of the source of
noise, usually an impossible task.
In more detail, one dimensional noise propagation allows ANC to occur
if the widest dimension of the noise channel (e.g. inside an air duct)
less than 1/4 wavelength. Therefore ANC in a duct one foot (0.3m) wide
not work above 275 Hz. Fortunately, such duct noise reduction is a
useful capability since noise
above 275 Hz is more easily absorbed by duct linings while noise below
275 Hz is
not, requiring some other means. ANC satisfies this need. The DIGISONIX
company (Wisconsin) has supplied such duct sound attenuation devices in
A case of 3-D propagation, but with the cancellation very near to the
reception point, is the ANC headset popular for pilots and other
noise-field situations where it is acceptable to wear an earmuff type
hearing protector. The fractional wavelength proximity requirement is
precisely achieve up to a frequency of about 500 Hz. Noise cancellation
truly outstanding at low frequencies down to 20 Hz or less (inside the
cup only - at your ear), depending on the quality of ear cup seal
your ear. SENNHEISER, DAVID CLARK, BOSE, HEADSETS (a kit), TELEX,
and NOISEBUSTER are typical suppliers.
The volume from which noise from common noise
sources such as engines, transformers, exhaust fans, and loud outdoor
music, emerges from is much more than a
quarter wavelength across. (The "Emission" Problem.) Here, a single
loudspeaker ANC transducer
to that noise source will not work because of that extent. A multiple
loudspeaker ANC system
can be built that allows noise cancellation in only one specific
Two common 3-dimensional
noise problems are traffic noise arriving from
a not-too-distant roadway, and noise already found inside a noisy room.
(The "Immission" Problem.) Here, the 1/4 wavelength distance
limit causes the effective volume where sound is canceled only in the
immediate vicinity of
the ANC loudspeaker. If the offending frequency is low (e.g. turboprop
noise at a passenger seat), noise cancellation of the propeller throb
100-200 Hz) is effective at a person's head position as long as that
person does not
move his or her head more than a few inches. Recent engineering efforts
are extending this volume with multiple loudspeakers. In
addition some attempts at driving the aircraft skin directly with
piezoelectric transducers cemented to the aircraft skin are under way.
Inquiries on the feasibility of ANC for noise (by the highway and at
windows) have been received recently. I have not given the inquirers
that this is feasible. If any one knows of progress in this matter, it
should be reported in the alt.sci.physics.acoustics
news group. (I know of
only one experiment on this, conducted several years ago by the
DOT, no results were reported to my knowledge.)
more contemporary information, investigate the following links:
perform a new web search on the subject:
Last updated 20-July-2007.
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Click Software, Inc.